So what's the etiquette if you mow down a moggy in your car?

I saw someone being very scathing about a motorist the other day who "callously drove away leaving it there" They were so angry at the "level of some people's cruelty ".

So I was questioning if this really was evidence of cruelty and callousness; perhaps they just didn't know what to do.

So are you supposed to scrape it up and put it at the roadside, or carry around the nearby houses asking if it belongs to anyone? I'm not a pet owner myself so I'm perhaps not up to speed with current Road-Kill practices; can any offer advice?